1. Field of the Invention
The present invention generally relates to improvements in automatic train-line air brake pressure monitoring systems and, more particularly, to a modification of such systems which replaces the reset or release circuits with circuits that emulate an automatic brake valve and cause an automatic brake manifold release whenever a four psi increase in pressure is sensed.
2. Description of the Prior Art
In U.S. Pat. No. 4,361,825, there is disclosed an automatic train-line air brake pressure monitoring system which includes a peak detector and hold circuit that determines the peak voltage of an input signal representing the air pressure in the air brake system. More specifically, an auto-referencing peak detector circuit is periodically reset prior to each pressure detection by a subtraction circuit such that the pressure is always measured with respect to the last measured air brake pressure value. The automatic monitoring system provides output signals on a continuous basis representative of the elapsed time air brakes are applied or released, as well as an indication of the quantity of the braking effort.
Railroad air brake systems typically operate in a released or off position with a compressed air pressure within a range of typically 70 psi to 100 psi. Application of the brakes is caused by a reduction in the air pressure on the brake pipe to all the cars. The more air pressure that is removed from the brake pipe, the greater the braking effort. An emergency braking application is provided if the air pressure drops below a certain specified minimum value.
In accordance with the invention disclosed in U.S. Pat. No. 4,361,825, whenever air pressure is reduced within the brake pipe system, the pressure drop from the maximum is measured and quantized, and an output indicative thereof is provided. The number of steps desired to be recorded, that is the number of pressure reductions, and the pressure differential setting for each step can be varied and programmed into the system. The air brake monitoring system also provides means for providing signals indicative of a release of the air brakes, as would occur with an increase in air pressure in the brake line. An emergency output signal can be provided indicative of a reduction of air pressure within the brake line below a predetermined threshold value.
The peak detector holds or stores a maximum detected voltage, proportional to air brake pressure, which is subtracted from the output of a transducer indicating the current air pressure in the brake line. Under normal conditions, that is under a brake released or off condition of the brake system, the resulting voltage output of the subtractor is zero and this provides a signal representing a "brakes released" condition. A drop in the detected pressure of the air line causes a negative signal at the output of the subtractor. When the magnitude of this pressure drop exceeds a pre-set threshold voltage, a "brakes applied" trigger signal output is provided to the monitoring system. The trigger signal output also resets the peak detector to the presently detected transducer voltage, such that the subtractor again has a zero voltage output. Such a process is repeated for each step of brake application.
When the brakes are released, the pressure transducer provides an increasing voltage output, and the voltage output of the subtractor therefore goes negative. When such negative signal exceeds a pre-set threshold, the output signal of the monitoring system provides a "brakes released" signal condition. The output signals of the automatic air pressure train-line monitoring system disclosed in U.S. Pat. No. 4,361,825 can be utilized to actuate a digital or analog recorder to provide a permanent record of the monitoring system outputs.
An alternate air brake monitor (ABM) system has been successfully implemented using a digital peak detector circuit. Similarly to the air brake pressure monitoring system disclosed in U.S. Pat. No. 4,361,825, in the digital ABM system whenever air pressure is reduced within the brake pipe system, the pressure drop from the maximum is measured and quantized, and an output indicative thereof is provided. The digital peak detector holds or stores a maximum quantized value which is proportional to air brake pressure. This quantized value is subtracted from the quantized output value from a transducer to indicate the current air pressure in the brake line. The output of the subtractor is zero under normal conditions indicating a "brake released" condition. A drop in the detected pressure of the air line results in a nonzero output with a negative sign from the subtractor. This nonzero output is compared in a plurality of comparators, each with an increasingly larger stored value, and when the nonzero output exceeds the first stored value, a "brakes applied" signal output is provided to the monitoring system. As the nonzero output exceeds the stored values for the other comparators, outputs from those comparators are used to provide signals that provide a digital indication of the progression of the application of the brakes. When the brakes are released, the pressure transducer provides an increasing voltage output which, when quantized, results in a smaller nonzero output from the subtractor. When that nonzero output falls below the first stored value, a "brakes released" condition is signaled.
While the prior automatic brake monitoring systems have functioned well in most situations, certain problems in operation have been experienced. The prior systems typically reset only on increasing pressure from either six or fourteen psi reduction or from release of an emergency condition. A problem in the operation arises when the locomotive pressure regulating feed-valve is changed. For example, if a locomotive comes from a coal train set at 110 psi, cuts off the train, reduces the feed-valve to 75 psi, and then connects to another train, the automatic brake monitoring system "stores" a 35 psi reduction. This causes the system to show a full application of air brakes so that it cannot be reset without an emergency condition being imposed. The failure of the system to properly reset can result in erroneous recordings showing continuous reductions throughout the recording tape.
Additionally, trailing units in a locomotive consist may or may not have their PC switch reset after an emergency condition. The PC switch is a device monitored by the ABM which closes on emergency or full service brake application. If it does not reset, the locomotive still operates but the automatic brake monitor "sticks" in "PC Switch Open" condition and over-rides the other functions of the ABM.
Premature "release" indications are also possible if the magnitude of a brake application is close to the preset threshold set points. Since the ABM senses the brake pipe pressure near the exhaust end of a string of railroad cars containing a volume of pressurized air and limited flow capacity, the pressure drops below the average pipe pressure and increases when the exhaust is cut off. Hence, a reduction in pressure having a value close to the preset threshold value may be sensed and recorded, followed by an erroneous release produced by a pressure value just above the threshold. For example, if the pressure at the automatic brake monitor is sensed as a 14.2 psi reduction during the brake reduction, followed by 13.8 psi reduction when the flow stops, the automatic brake monitor will reset and show a "released" condition if the threshold is 14.0 psi.